How can we classify earthquakes based on their depth of focus?

Earthquakes can be classified into three types based on their depth of focus: shallow, intermediate, and deep-focus earthquakes.

Shallow-focus earthquakes are the most common type, accounting for about 75% of all seismic activity. These earthquakes occur at depths less than 70 kilometres below the Earth's surface. They are typically the most destructive because the epicentre, the point on the Earth's surface directly above the focus, is close to the surface. This proximity means that the seismic waves have less distance to travel before they reach the surface, resulting in stronger shaking and potentially more damage.

Intermediate-focus earthquakes occur at depths ranging from 70 to 300 kilometres. They account for approximately 20% of all earthquakes. The seismic waves from these earthquakes have to travel a greater distance to reach the surface, which can reduce their destructive power. However, they can still cause significant damage, particularly if they occur near populated areas.

Deep-focus earthquakes are the least common, making up about 5% of all earthquakes. These occur at depths greater than 300 kilometres, sometimes reaching as deep as 700 kilometres. Despite their depth, these earthquakes can still be felt at the surface, although their effects are usually less severe than those of shallow or intermediate-focus earthquakes. The mechanism for deep-focus earthquakes is not fully understood, as the high pressure and temperature at such depths should inhibit the kind of frictional sliding usually associated with earthquakes.

The depth of an earthquake's focus is determined by the tectonic activity that causes it. Shallow-focus earthquakes tend to occur at divergent and transform boundaries, where tectonic plates are moving apart or sliding past each other. Intermediate and deep-focus earthquakes are more common at convergent boundaries, where one tectonic plate is being forced under another in a process known as subduction. The descending plate can cause earthquakes as it pushes deeper into the Earth's mantle.

In conclusion, the depth of an earthquake's focus is a key factor in determining its potential for destruction. By understanding the different types of earthquakes and the tectonic processes that cause them, we can better predict and prepare for these natural disasters.